This invention relates generally to fasteners, and more particularly to fasteners of the type wherein a bolt member having grooves therein and a swage collar are used to fasten a member to a workpiece.
Fasteners of this type generally have a series of grooves formed in the shank of the bolt member. The bolt is then passed, for example, through holes in two workpieces which are desired to be secured together. The collar is then placed over the grooved end of the bolt which has passed through the workpiece and is swaged into the grooves on the shank. In order to tightly secure the two workpieces together, the bolt shank is placed in tension. The tension is reacted to at one end of the shank by the bolt head which abuts one of the workpieces and the collar which abuts the other workpiece. The load is transferred from the load bearing surfaces or shoulders formed by the grooves in the shank to the mating load bearing surfaces which are formed in the collar when it is swaged into the grooves of the shank.
It is well known that in conventional grooved fasteners, such as shown, for example, in U.S. Pat. No. 3,645,125, a very high proportion of the load is carried by the load bearing surfaces formed by the first few grooves in the shank adjacent the sheet line of the workpiece. Experience has shown that most structural failures occur in this area of the shank or in the mating region of the collar.
Although this problem has generally been recognized, there are few solutions to the problem disclosed in the prior art. One solution which has been suggested for solving the problem is to increase the distance between and radial depth of each adjacent groove as the distance from the sheet line increases, such as in U.S. Pat. Nos. 2,396,661 and 2,397,076. This configuration, however, is relatively expensive to manufacture and inspect after manufacture.